This invention relates generally to a cooking magnetron system and, more particularly, to such a system having an inverter with feedback control.
Today commercially available domestic microwave ovens commonly employ microprocessor based electronic controls. Such controls typically include an entry keyboard, and a display device, each operatively connected to a microprocessor. The microprocessor contains memory which sequences the correct operation of the microwave oven following a command or request entered by the user via the keyboard. Typically, the electronic controls may display the time of day, cooking mode, cooking time selected, cooking time remaining, selection of special cooking algorithms, power level selected, food temperature selected, and food temperature measured. The microprocessor receives data from the keyboard, monitors door closure, and receives temperature data from a temperature probe or meat thermometer. Additionally, the microprocessor may control the power level by duty cycle switching of a power transformer as used in a 50 or 60 Hz LC power supply system. The electronic controls including the microprocessor may also function to turn on a cooling fan, cavity illumination lamp, and generate an audible warning or alarm.
Although the microprocessor based microwave oven control systems have been generally useful, the microcomputer controls commonly used for appliances have not been suitable for some types of power supplies. For example, typical microcomputer or microprocessor appliance control arrangements do not provide sufficiently fast control for controlling a high frequency switching inverter such as is disclosed in the hereinbefore referenced U.S. patent application Ser. No. 138,138 which operates at a switching frequency on the order to 20-30 KHz. Commonly used low cost appliance type controllers such as Texas Instruments TMS 2XXX series of 4 bit microcomputers or the generally similar Hitachi type MMCS 43 or HMCS 44A do not generate voltage logic pulses at a sufficiently high rate to use the microcomputer output for controlling the inverter frequency. Such controllers may take longer to fetch a single instruction and to cause the transfer of data than the maximum pulse width required if the inverter were to be directly driven. Limitations, such as timing considerations, therefore, pose significant problems for microprocessor control of some microwave arrangements.
The hereinbefore referenced patent application, Ser. No. 138,137 discloses and claims a system which overcomes the problems presented by timing considerations for controlling the output power of an inverter driven magnetron by indirectly coupling the microprocessor to the inverter, via a relatively slow feedback loop for power control purposes. This arrangement works satisfactorily for power control, filament regulation and other control functions that can tolerate time delays on the order of hundreds of milliseconds. In fact, in control functions such as power control such delays can be beneficial in preventing or limiting overshoot by the controlled parameter.
However, in a power supply circuit comprising a full wave bridge inverter, the semiconductor switching devices are switched on and off in phased pairs very rapidly with rise and fall times many times faster than the clock frequency of the power control microprocessor. The fast rise and fall times are necessary to reduce the power dissipated in the switching devices. Additionally, in order to protect the switching devices from damaging overcurrent conditions, a relatively fast response time is needed to respond to such fault conditions before overload damage occurs. Slow response time is ill-suited for protecting against excessive current in a microprocessor controlled inverter having a frequency of, for example, 15 to 30 KHz.
Therefore, a need exists for a control arrangement for a high frequency inverter driven magnetron power control system which enjoys the advantage of microprocessor control and which is adequately protected against faults requiring response times faster than that achievable with a microprocessor.